1. Technical Field
The invention relates to devices and methods for cleaning the interior surfaces of either a chemical vapor deposition (CVD) reactor or etch reactor using a plasma etch process to remove unwanted depositions.
2. Background Art
Reactors can perform various semiconductor processes on semiconductor substrates (often referred to as wafers), including dielectric etching and chemical vapor deposition (CVD). A reactor can employ purely chemical processes to perform CVD or etching, or else (depending upon the type of reactor), can perform radio frequency (RF) plasma CVD or plasma etching. The present description does not concern the process employed to carry out etching or CVD on a wafer but rather concerns a process for cleaning the interior surfaces of the reactor. Typically, during processing, unwanted depositions form on the interior surfaces of the reactor's vacuum chamber. For example, in a reactor used to perform chemical vapor deposition of silicon dioxide films, silicon dioxide (SiO.sub.2) residue forms over the interior surfaces of the chamber. In RF plasma CVD reactors having RF electrodes inside the chamber, the electrodes themselves can become contaminated with silicon dioxide. Other materials can contaminate the chamber interior surfaces, depending upon the type of process performed by the reactor. For instance, silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), and various silicon oxynitride (Si.sub.x O.sub.y N.sub.z) deposits are also possibilities.
Unwanted depositions such as those described above, if allowed to build up on the interior surfaces of the reactor chamber, can inhibit the performance of the reactor. For example, in a CVD reactor, such unwanted depositions change the deposition rate from one wafer to the next, reduce the deposition uniformity across the surface of a given substrate, and change the layer stresses so that a consistent stress level cannot be maintained between successive thin film layers. In addition, these depositions could eventually flake off as conditions within the reactor chamber change during processing, thereby forming particulates. These particles may fall onto the wafer being processed, thereby damaging devices formed thereon and reducing the yield.
Preferably, the foregoing difficulties are generally avoided by cleaning the reactor chamber interior shortly before inserting a wafer to be processed into the chamber. In a reactor equipped with RF plasma excitation electrodes and/or an RF plasma excitation antenna, silicon dioxide deposits are typically cleaned by the introduction of an etchant gas, such as ethyl hexafluoride (C.sub.2 F.sub.6), into the chamber. The etchant gas is ignited into a plasma by the RF excitation apparatus of the reactor. For high-rate CVD RF plasma reactors, an RF excitation field of 13.56 MHz is typically employed to ignite the plasma and is used during both wafer processing and chamber clean operations. In fact, a 13.56 MHz RF field with C.sub.2 F.sub.6 etchant gas is the most often used combination in performing RF plasma cleaning operations involving silicon dioxide deposits. The C.sub.2 F.sub.6 gas, when ignited as a plasma, produces a radical CF.sub.3 in the following reaction: EQU C.sub.2 F.sub.6 +energy from plasma.fwdarw.2CF.sub.3,
the CF.sub.3 providing the fluorine reactant species for etching of silicon dioxide deposits.
In the case of silicon nitride or silicon oxynitride deposits, the cleaning operation is typically performed using carbon tetrafluoride (CF.sub.4) or carbon hexafluoroethane and the like, along with an oxygen source such as oxygen or nitrous oxide and the like.
One problem with current cleaning processes is that the unwanted deposits on the interior chamber surfaces typically vary in thickness from one location to the next. Therefore, it is difficult to remove the unwanted deposit uniformly. Some of the interior chamber surfaces will still have remaining deposits, when others have been completely cleaned. One solution to this problem would be to perform the cleaning process for a long enough period of time to ensure all the deposits are removed. However, this would further reduce the productive cycle of the reactor, forcing it to spend more time in the unproductive cleaning process rather than in processing wafers. In addition, prolonged plasma cleaning operations can result in significant damage to cleaned chamber surfaces. The extra time required to ensure that all the chamber surfaces are thoroughly cleaned can cause varying degrees of damage to the first-cleaned surfaces.
Another problem with the use of C.sub.2 F.sub.6, and related etchant gases such as CF.sub.4, is that they are so-called "greenhouse" gases. The U.S. Environmental Protection Agency has ordered the reduction of all such gases which contribute to the depletion of the ozone layer and support the greenhouse effect. This means limiting or curtailing the use of the most widely employed etchant gases for reactor chamber cleaning.
Of course, other "non-greenhouse" gases are available to perform the aforementioned cleaning operation. For example, nitrogen-fluorine based etchant gases, such as NF.sub.3, provide one alternative. NF.sub.3 is a very aggressive etchant gas because it produces more fluorine in the plasma than the aforementioned "greenhouse" gases. Therefore, the interior surfaces of the reactor chamber are cleaned much faster. However, there are drawbacks to using NF.sub.3. One characteristic property of NF.sub.3 is that it is strongly electronegative (more so than CF.sub.4 or C.sub.2 F.sub.6) and therefore tends to reduce the population of free electrons in the plasma, due to its great affinity for electrons. The problem is that at RF excitation frequencies in the megahertz range (such as, for example, 13.56 MHz), a depleted electron population causes insufficient coupling between the RF signal and the plasma to sustain the plasma. This is primarily because at such high frequencies, the electrons are the only charged particles in the plasma with sufficient charge-to-mass ratio to be able to follow the rapid RF field oscillations. Thus, it is the electrons which kinetically couple the energy from the RF field to the ions and radicals in the plasma. The result is that as NF.sub.3 begins to reduce the electron population in the plasma, portions of the plasma become unstable, and flickering or collapsing of the plasma may be observed. Such plasma instability occurs at temperatures below 1000 degrees C and at chamber pressures between 0.005 and 10 Torr and at RF excitation frequencies on the order of 13.56 MHz. However, these are the very parameters preferred for current plasma cleaning operations. Plasma instabilities make the cleaning process uncertain, since plasma flickering results in the plasma density being reduced in various regions of the chamber for varying durations. This makes cleaning process control problematic, and the chamber may not be entirely cleaned at the conclusion of a given chamber cleaning process. With such instabilities, the effective plasma "on" time throughout the chamber is uncertain and so it is not possible to predict the required clean time with reasonable accuracy for consistent repetitive chamber clean operations. As a result, the specified chamber clean time is typically increased above that which would normally be necessary if the plasma was stable. This increased cleaning time can result in the damage to the first-cleaned chamber surfaces, as discussed previously, as well as increased processing downtime. And finally, it is noted that NF.sub.3 is a relatively expensive gas, as compared to other etchant species which are employed in plasma cleaning operations, thus making its use costly.
Another example of a "non-greenhouse" gas that could be employed for chamber cleaning is sulfur hexafluoride (SF.sub.6). However, when this etchant gas is used at the typical pressures and excitation frequencies, it exhibits an extremely poor cleaning performance. Due to this inefficiency, large quantities of the gas are required, along with long cleaning times. Thus, the expense of using this gas is high and wafer throughput is low.
As is evident from the above discussion, devices and methods for cleaning the interior surfaces of a plasma CVD or etch reactor are needed which will reduce or eliminate the need to employ "greenhouse gases", and alleviate the problems associated with the aforementioned alternate etchant gases. In addition, it is desired to reduce the cleaning times, without sacrificing the thoroughness of the cleaning, so as to improve the throughput of the plasma reactor.